Diode pumped lasers have grown in usefulness, particularly in industrial, medical and military applications. Diode pumped lasers are particularly useful in that diode pumps are power efficient, all solid-state and long lived. This results in laser systems that are lighter, more efficient and typically not water cooled, as compared to similar flashlamp pumped solid-state lasers.
As is shown in FIG. 1, most conventional diode-pumped lasers use an “end-pumped” configuration where the laser diode 1 pump light is introduced into the lasing medium 2, via collimating/focusing optics 3, along the lasing axis. The laser medium 2 is contained within a resonant cavity structure defined by an input/high reflector mirror 4 and an output coupler 5. This configuration is useful for generating efficient lasing with excellent beam quality. Diode end pumped lasers also typically use only one diode pump source and, owing to the fact the diode lasers are usually the most expensive component, this configuration maximizes the use of available pump light. Diode pumps that are used for end pumping are usually either directly collimated and focused into the gain medium 2, as shown in FIG. 1, or the pump light is introduced from the end of an optical fiber 6, as shown in FIG. 2. Arrays or single emitters, used in this fashion, have sufficient brightness or beam quality to enable efficient end-pumping.
An example of an end pumped microlaser is provided in U.S. Pat. No. 5,394,413 “Passively Q-Switched Picosecond Microlaser” issued Feb. 28, 1995 to Zayhowski. This patent discloses a microlaser for producing high-peak-power pulses of light of short duration. The short duration is achieved by controlling factors such as the length of the cavity. Accordingly, the microlaser is not scaleable, by design. As with some other lasers, the microlaser makes use of diodes for pumping.
Diode arrays used for end pumping are typically not the highest power devices, having peak powers ranging from a few watts. Some may be fiber coupled to achieve powers up to 40 watts. The highest power devices, known as quasi-cw diode bars, with peak powers up to 100 W per device, can also be configured into stacked arrays 7 which are capable of up to kW of peak power. These devices are used in a pulsed configuration, and due to reduced brightness, are coupled into the lasing medium transverse to the laser mode, or in a so-called side-pumping configuration as shown in FIG. 3.
Using high-power, quasi-cw diode arrays 7 in the side pumping configuration is typically most useful for generating high peak-power pulses in the pulse energy regime of millijoules to Joules. Diode side-pumped lasers are usually Q-switched either electro-optically or passively using a saturable absorber as a Q-switch 8. There are many side pumped, Q-switched laser embodiments published in the literature, with many of the proposed architectures emphasizing performance. In diode side-pumped geometries, the gain media is typically either a rod or a slab. Slab geometries have typically been used in conductively cooled laser systems with one side of the slab attached to a thermal heat sink, and with the opposing face used for the introduction of pump light. This asymmetric pumping geometry inevitably leads to uncompensated thermal gradients which result in lensing, stress induced birefringence and other optical aberrations. Side pumped slabs can employ various techniques such as utilizing a so called “zig-zag” optical path. Zig-zag slabs, however, are difficult to fabricate owing to tight optical tolerances and are therefore more difficult to produce in large quantities than straight through slab embodiments, and are therefore more expensive to produce.
One example of a laser using a zig-zag optical path is disclosed in U.S. Pat. No. 6,377,593 “Side Pumped Q-Switched Microlaser and Associated Fabrication Method” issued Apr. 23, 2002 to Peterson et al. This patent discloses Q-switched microlaser that uses a zig-zag resonation pattern with side pumping of the active gain medium so as to effectively lengthen the microresonator cavity without physically lengthening the microresonator cavity. In addition to imposing certain requirements upon the orientation of the end faces, this patent teaches use of both reflectance and anti-reflection coatings upon the sides of the microresonator, such techniques being more costly to manufacture and control.
Another design is presented in the publication “Monolithic Self-Q-Switched Cr,Nd:YAG Laser”, S. Zhou et al., Optics Letters, Vol. 18, No. 7, pgs. 511–512, Apr. 1, 1993. This publication describes a monolithic laser that end pumps a codoped Cr,Nd:YAG crystal with the focused output of a diode laser. Zhou et al. later obtained a patent for a “Monolithic Self Q-Switched Laser” on May 9, 1995 (U.S. Pat. No. 5,414,724). This patent discloses a laser that includes a length of solid-state laser material with a plurality of dopants (co-doping), so that the material can generate coherent radiation for laser action and, in the same material, provide saturable absorption at the wavelength of the laser emission necessary for Q-switching. This laser suffers from certain drawbacks. For example, manufacturing the laser material requires careful control of the amount of co-doping to ensure proper growth of the crystal. Failure to ensure proper controls can lead to rejection of the microlaser during manufacturing. Further, as the saturable absorber material is present at the entrance of the pump light, it is available to interfere with the pump light, and therefore cause performance issues.
“Diode-Pumped Passively Q-Switched Picosecond Microchip Lasers”, J. J. Zayhowski et al., Optics Letters, Vol. 19, No. 18, pgs. 1427–1429, Sep. 15, 1994, describes an end pumped laser where the gain medium was a 0.5 mm long piece of Nd3+:YAG crystal, where the saturable absorber (Q-switch) was a 0.25 mm long piece of Cr4+:YAG, and where the output of a pump diode was coupled to the end of the gain medium through 100 micron core optical fiber.
“Single-Mode High-Peak-Power Passively Q-Switched Diode-Pumped Nd:YAG Laser”, R. S. Afzal et al., Optics Letters, Vol. 22, No. 17, pgs. 1314–1316, Sep. 1, 1997, describes linear and ring-cavity laser configurations where a Nd:YAG seven bounce slab is side pumped using a close coupled 1 cm long 100 W quasi-cw diode pump array within, for the linear embodiment, a 5 cm long resonator structure bounded by a flat output coupler and a 2.5 m radius of curvature high reflector.
“Monoblock Laser For A Low-Cost, Eyesafe, Microlaser Range Finder”, J. E. Nettleton et al., Applied Optics, Vol. 39, No. 15, pgs. 2428–2432, May 20, 2000, describes a flash-lamp-pumped Nd3+:YAG crystal with a Cr4+:YAG passive Q-switch and an intracavity potassium titanyl arsenide (KTA) optical parametric oscillator (OPO) used for frequency conversion from 1.06 microns to 1.54 microns. A feature of this design is that the 1.54 micron laser cavity consists of four rectangularly shaped crystals: the Nd:YAG laser rod (25 mm long); a Nd:YAG endcap (10 mm long), where the laser rod and endcap have complementary end faces cut at the Brewster angle; the Cr4+:YAG passive Q-switch (3 mm long); and the KTA OPO. These four components were arranged on and bonded to a ceramic laser pallet, but not to one another, and in combination with a commercially available instant camera flashtube, formed a monoblock laser transmitter for a range finder.
Reference with regard to an example of a diode-side-pumped laser may be had to U.S. Pat. No. 5,485,482, “Method for Design and Construction Of Efficient, Fundamental Transverse Mode Selected, Diode Pumped, Solid State Lasers”, issued Jan. 16, 1996 to M. D.
Selker and R. S. Afzal.
In general, Q-switched micro-lasers or monolithic lasers are end-pumped, such as those described in U.S. Pat. Nos. 5,394,413, 5,381,431, 5,495,494, 5,651,023, and 6,373,864 B1. U.S. Pat. Nos. 6,219,361 B1 and 6,377,593 B1 describe side pumped designs, where the beam path takes an internal zig-zag path, such designs lengthening the optical pulse and increases manufacturing difficulty and cost.
What is needed is a scaleable microlaser that is low cost and simple to manufacture. Preferably, the laser should incorporate techniques for ensuring reliable and high performance operation during thermal loading.